Coordinated Tbx3/Tbx5 transcriptional control of the adult ventricular conduction system

The cardiac conduction system (CCS) orchestrates the electrical impulses that enable coordinated contraction of the cardiac chambers. The T-box transcription factors <i>TBX3</i> and <i>TBX5</i> are required for CCS development and associated with overlapping and distinct human CCS diseases. We evaluated the coordinated role of <i>Tbx3</i> and <i>Tbx5</i> in the murine ventricular conduction system (VCS). We engineered a compound <i>Tbx3:Tbx5</i> conditional knockout allele for both genes located in cis on mouse chromosome 5. Conditional deletion of both T-box transcriptional factors in the VCS, using the VCS-specific <i>MinK^CreERT2</i>, caused loss of VCS function and molecular identity. Combined <i>Tbx3</i> and <i>Tbx5</i> deficiency in the adult VCS led to conduction defects, including prolonged PR and QRS intervals and elevated susceptibility to ventricular tachycardia. These electrophysiological defects occurred prior to detectable alterations in cardiac contractility or histologic morphology, indicative of a primary conduction system defect. <i>Tbx3:Tbx5</i> double-knockout VCS cardiomyocytes revealed a transcriptional shift toward non-CCS-specialized working myocardium, indicating a change to their cellular identity. Furthermore, optical mapping revealed a loss of VCS-specific conduction system propagation. Collectively, these findings indicate that <i>Tbx3</i> and <i>Tbx5</i> coordinate to control VCS molecular fate and function, with implications for understanding cardiac conduction disorders in humans.